yes you can as its so vigouress excuse my spelling . i think and its only my opinion lol just take branches out that are full grown so it doesnt just look like a bush . it would be happier if the inside was more airy and sunny . cut any branches out that are rubbing/touching another branch to start with . realy the acre is happy as it is so you cut it for you . evenly take a branch here and there and keep stepping back as often as you like till you like it . early spring or late autumn when its dorement would be better . i think it would be ok now realy looking at it . if you just want it smaller ie as it looks but a general trim or short back and sides lol just do it as i have an acer shrub/tree that was a bonzei for 16 years . i planted it and it grows realy long branches and get leggy so i cut back as and when but its fine .